1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting wire, and more particularly, it relates to a method of preparing an oxide superconducting wire of a multi-filamentary structure comprising a step of charging raw material powder for an oxide superconductor into a metal tube.
2. Description of the Background Art
In general, it is known that the following steps are employed for preparing an oxide superconducting wire of a multi-filamentary structure by coating an oxide superconductor containing copper with a metal:
(i) charging raw material powder for an oxide or the like into a metal tube consisting of silver or the like and thereafter drawing the metal tube thereby preparing a single-filamentary material;
(ii) bundling a plurality of such single-filamentary materials and charging the bundle into another metal tube for forming a base material of a multi-filamentary structure; and
(iii) performing drawing, rolling and the like on the base material of the multi-filamentary structure for forming a wire and thereafter heat treating the wire for sintering the raw material powder thereby obtaining an oxide superconducting wire.
In such a method of preparing an oxide superconducting wire, factors of the raw material powder charged into the metal tube and the performance (critical current) of the oxide superconducting wire are strongly correlated with each other. In particular, the types and the ratios of the elements forming the raw material powder consisting of CaO and CuO or having the structure of CaCuO2, for example, the sizes (grain sizes) of the grains forming the powder and the like can be exemplified as the factors of the raw material powder.
Some reports have been made on studies noting the sizes of the grains forming the raw material powder among these factors. While the factor of the sizes of the grains forming the raw material powder is grasped in view of the mean grain size in these reports, no consideration is made on the correlation between the sizes of the grains forming the raw material powder and the shape of the wire (for example, the size of superconductor filaments included in the wire). In general, therefore, it is difficult to maximally bring out the performance of the oxide superconducting wire.
An object of the present invention is to clarify the relation between the sizes of grains forming raw material powder charged into a metal tube and the size of superconductor filaments included in a wire and provide a method of preparing an oxide superconducting wire capable of maximally improving the critical current of the oxide superconducting wire on the basis of the relation.
The inventor has made various studies on the relation between the sizes of grains forming raw material powder charged into a metal tube and the size of superconductor filaments included in a wire, to find out that it is necessary to select the optimum grain size of the raw material powder charged into the metal tube not with reference to the mean grain size but with reference to the maximum grain size while adjusting the selected optimum maximum grain size in response to the diameter of the superconductor filaments included in the prepared wire, in order to increase the critical current of the oxide superconducting wire. The present invention has been proposed on the basis of the aforementioned recognition of the inventor.
A method of preparing an oxide superconducting wire according to an aspect of the present invention comprises the following steps (a) to (g) and feature (h):
(a) preparing raw material powder for an oxide superconductor;
(b) charging the raw material powder into a first metal tube;
(c) reducing the diameter of the first metal tube charged with the raw material powder thereby obtaining an elementary wire;
(d) charging a plurality of such elementary wires into a second metal tube;
(e) reducing the diameter of the second metal tube charged with the elementary wires thereby obtaining a first wire, isotropically shaped in cross section, having a plurality of first filaments each containing the raw material powder;
(f) performing uniaxial compression on the first wire thereby obtaining a tape-like second wire having a plurality of second filaments each containing the raw material powder; and
(g) performing heat treatment on the second wire thereby obtaining an oxide superconducting wire including a plurality of superconductor filaments, and
(h) the maximum grain size of the raw material powder is smaller than the minor diameter of the first filaments.
In the aforementioned method of preparing an oxide superconducting wire, the maximum grain size of the raw material powder is preferably smaller than the minor diameter of the second filaments.
The aforementioned method of preparing an oxide superconducting wire preferably further comprises a step of performing uniaxial compression on the second wire subjected to the heat treatment and thereafter performing heat treatment.
A method of preparing an oxide superconducting wire according to another aspect of the present invention comprises the following steps (i) to (q) and feature (r):
(i) preparing raw material powder for an oxide superconductor;
(j) charging the raw material powder into a first metal tube;
(k) reducing the diameter of the first metal tube charged with the raw material powder thereby obtaining an elementary wire;
(l) charging a plurality of such elementary wires into a second metal tube;
(m) reducing the diameter of the second metal tube charged with the elementary wires thereby obtaining a first wire, isotropically shaped in cross section, having a plurality of first filaments each containing the raw material powder;
(n) charging a plurality of such first wires into a third metal tube;
(o) reducing the diameter of the third metal wire charged with the first wires thereby obtaining a second wire, isotropically shaped in cross section, having a plurality of second filaments each containing the raw material powder;
(p) performing uniaxial compression on the second wire thereby obtaining a tape-like third wire having a plurality of third filaments each containing the raw material powder; and
(q) performing heat treatment on the third wire thereby obtaining an oxide superconducting wire including a plurality of superconductor filaments, and
(r) the maximum grain size of the raw material powder is smaller than the minor diameter of the second filaments.
In the aforementioned method of preparing an oxide superconducting wire, the maximum grain size of the raw material powder is preferably smaller than the minor diameter of the third filaments.
The aforementioned method of preparing an oxide superconducting wire preferably further comprises a step of performing uniaxial compression on the third wire subjected to the heat treatment and thereafter performing heat treatment.
In the aforementioned method of preparing an oxide superconducting wire according to this aspect, the uniaxial compression is preferably rolling.
A method of preparing an oxide superconducting wire according to still another aspect of the present invention comprises the following steps (s) to (x) and feature (y):
(s) preparing raw material powder for an oxide superconductor;
(t) charging the raw material powder into a first metal tube;
(u) reducing the diameter of the first metal tube charged with the raw material powder thereby obtaining an elementary wire;
(v) charging a plurality of such elementary wires into a second metal tube;
(w) performing plastic working on the second metal tube charged with the elementary wires thereby obtaining a tape-like wire having a plurality of raw material filaments each containing the raw material powder; and
(x) heat treating the tape-like wire thereby obtaining an oxide superconducting wire including a plurality of superconductor filaments, and
(y) the maximum grain size of the raw material powder is smaller than the minor diameter of the raw material filaments.
In the method of preparing an oxide superconducting wire according to this aspect, the oxide superconductor is preferably a bismuth-based oxide superconductor. The bismuth-based oxide superconductor is preferably a Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O based oxide superconductor.
In this case, the step of preparing the raw material powder for the oxide superconductor preferably prepares raw material powder including a non-superconducting phase and a superconducting phase containing (Bi, Pb), Sr, Ca and Cu in composition ratios of about 2:2:1:2. Each superconductor filament included in the oxide superconducting wire preferably includes a superconducting phase containing (Bi, Pb), Sr, Ca and Cu in composition ratios of about 2:2:2:3.
In the method of preparing an oxide superconducting wire according to this aspect, the metal tube charged with the raw material powder preferably consists of silver or a silver alloy.
According to the present invention, as hereinabove described, the critical current of the oxide superconducting wire can be maximally increased by preparing the raw material powder so that the maximum grain size of the raw material powder charged into the metal tube is smaller than the diameter of the superconducting filaments included in the prepared wire.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.